NEW: Nominations are now open for the 2019 Swift Community Awards! >>

What is function composition?

Swift version: 5.1

Paul Hudson    @twostraws   

Function composition is the ability to combine small functions together to make bigger functions.

In normal circumstances, you’d give some input to function A and get back output, and do the same for function B. When those two functions as combined together, A’s output becomes B’s input, so you provide input to A and get the results back from B.

To demonstrate this, here’s a function that generates random numbers in a specific range:

func generateRandomNumber(max: Int) -> Int {
    return Int(arc4random_uniform(UInt32(max)))

And here’s a function that spells out any number it’s given:

func spell(number: Int) -> String {
    let formatter = NumberFormatter()
    formatter.numberStyle = .spellOut
    return formatter.string(from: number as NSNumber) ?? ""

We can define a new operator that lets us combine those two together, like this:

precedencegroup CompositionPrecedence {
    associativity: left

infix operator >>>: CompositionPrecedence

func >>> <T, U, V>(lhs: @escaping (T) -> U, rhs: @escaping (U) -> V) -> (T) -> V {
    return { rhs(lhs($0)) }

You can learn more about how that works in Pro Swift.

You can now combine your two smaller functions into bigger ones, like this:

let spellOutRandom = generateRandomNumber >>> spell

spellOutRandom() is designed to take the input from the first function (a number) and send back the output from the second function (a string), so we can use it like this:


That will output a different spelled number each time it’s run.

SAVE 20% ON iOS CONF SG The largest iOS conference in Southeast Asia is back in Singapore for the 5th time in January 2020, now with two days of workshops plus two days of talks on SwiftUI, Combine, GraphQL, and more! Save a massive 20% on your tickets by clicking on this link.

Available from iOS 8.0 – learn more in my book Pro Swift

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!

Average rating: 1.0/5